1,720,977 research outputs found
Reductive Cleavage of N-Substituted 2-Aryl- l,3-oxazolidines: Generation of alfa-Amino-Substituted Carbanions
No full textThe behavior of several N-substibuted 2-aryl-1,3-oxazolidines has been investigated under conditions
of electron transfer from alkali metals in aprotic solvents. The reduction led to the regioselective
cleavage of the benzylic carbon-oxygen bond, with formation of the corresponding N-substituted
benzylamino alcohols in good yields. Investigation of the mechanism of this reductive cleavage, with
the aid of labeling experiments, showed the intermediate formation of a-tertiary amino-substituted
carbanions
Electron-transfer-induced reductive cleavage of phthalans: Reactivity and synthetic applications
No full textThe behavior of phthalan (1a) was investigated under conditions of electron transfer from alkali metals in aprotic solvents. Reaction with lithium in the presence of a catalytic amount of naphthalene in THF led to the reductive cleavage of an arylmethyl carbon-oxygen bond, with formation of a stable dilithium compound. Trapping of this intermediate with several electrophiles (alkyl halides, carbonyl derivatives, CO2) was successful. The extension of this procedure to several substituted phthalans (1b-i) was investigated, and the regiochemistry as well as the synthetic usefulness of these reactions are discusse
REGIOSELECTIVITY IN THE REDUCTIVE CLEAVAGE OF PYROGALLOL DERIVATIVES - REDUCTIVE ELECTROPHILIC SUBSTITUTION OF ACETALS OF 2,3-DIMETHOXYPHENOL
No full textAcetals of 2,3-dimethoxyphenol were used as the starting materials for the transformation of 1,2,3-trioxygenated benzenes into various 1-oxygenated-2,3-dicarbon-substituted benzenes, via regioselective reductive electrophilic substitution of the 2-methoxy group, followed by conversion into the corresponding triflates and a Pd-catalysed cross-coupling reaction. The regioselectivity of the reductive cleavage is ascribed to twisting of the leaving methoxy group out of the plane of the aromatic ring by the two ortho substituents. According to th is methodology, a new synthesis of lunularic acid is presented
Regioselective Reductive Electrophilic Substitution of 1,2,3-Trimethoxybenzene and Its 5-Alkyl-Substituted Homologues
No full textThe methoxy group in the 2-position of 1,2,3-trimethoxybenzene (1) can be regioselectively removed by electron
transfer from alkali metals and replaced with a variety of electrophiles in a one-pot procedure, affording 2-subtituted
resorcinol dimethyl ethers. The usefulness of this synthetic method is illustrated by numerous examples. This
reaction procedure has been successfully extended to the 5-methyl-substituted homologue (2), but limitations
occur with the higher homologue l-pentyl-3,4,5-trimethoxybenzen(e3 ). Investigations on the mechanism of
demethoxylation, with the aid of labeling experiments, provided clear evidence for the intermediacy of aryl radicals
and explained the low yields obtained in the reductive electrophilic substitutions of compound
Electron-transfer-induced reductive dealkoxylation of alkyl aryl ethers .2. Reductive cleavage of isomeric dimethoxybenzenes
No full textThe reactivity of the three isomeric dimethoxybenzenes in the reduction with potassium metal in THF and iso-octane in the absence of proton donors has been studied. The ortho and meta isomers reacted readily to afford products of both demethoxylation and demethylation, with the former prevailing for the ortho isomer in THF and for both isomers in iso-octane. Furthermore, in iso-octane, particularly at high temperature, significant amounts of benzene, the product of double demethoxylation, were formed. Exclusive demethylation was instead observed under all reaction conditions for the para isomer, which showed to be the least reactive of the three isomers. Labelling experiments have also been carried out in order to shed light on the mechanism of demethoxylation. The results are consistent with a competition between two modes of fragmentation of a radical anion, leading to aryl radical and methoxide anion on one side (demethoxylation) and to phenoxide anion and methyl radical on the other side (demethylation); in a subsequent step, the radicals thus formed abstract a hydrogen atom from components of the reaction medium or are reduced by single electron transfer to the corresponding anions. As concerns demethylation, however, a mechanism involving fragmentation of a dianion cannot be excluded. In light of previous results, a rationalization of the different reactivity of the three isomers is discusse
Reductive lithiation of arylalkyl methyl ethers
No full textWe have investigated the reductive cleavage of arylalkyl methyl ethers with an excess of lithium metal and a catalytic amount of naphthalene. The reaction proceeds regioselectively in the presence of various substituents on the aromatic ring, allowing access to a wide array of arylalkyl lithium derivatives, some of which are not easily accessible by conventional method
Electron-Transfer-Induced Reductive Dealkoxylation of Alkyl Aryl Ethers. III. Reductive Cleavage of Methoxy-Substituted N,N-Dimethylanilines
No full textThe reactivity of the three isomeric methoxy-substituted N, N-dimethylanilines (N, N-dimethylanisidines) and of N, N-dimethyl-2,6-dimethoxyaniline in the reduction with alkali metals in aprotic solvents was investigated. N, N-Dimethyl-p-methoxyaniline was found to be unreactive, while the other substrates underwent exclusive cleavage of carbon-oxygen bond(s), with the following order of reactivity: 2,6-dimethoxy>o-methoxy>m-methoxy>p-methoxy. Both the relative reactivity and the regioselectivity of cleavage (demethoxylation vs. demethylation) was found to parallel closely that of the corresponding di- and trimethoxy-substituted substrates. These results suggest that intermediates with different electron distribution or even different intermediates are involved in the reductive cleavage of aryl-oxygen and aryl-nitrogen bond
Wittig Rearrangement of Aromatic Acetals and Ketals Induced by Reductive Electron Transfer
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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